OpenSpecEngineering

Finds the smallest continuous and surge (starting) ratings a generator needs to carry a set of loads. Enter the total running watts of everything it will power, the running and starting watts of the largest motor, and a safety margin. Homeowners, contractors and facility staff use it to pick a portable or standby generator before comparing models.

Equations

P_cont = P_run·(1 + m/100)

P_surge = [P_run + (P_start − P_motor)]·(1 + m/100)

P_run is the total running load, with every load that may be on at once added up. P_motor is the running power of the largest motor, which is already part of P_run. P_start is that motor’s starting surge, the brief demand while it comes up to speed. m is the safety margin in percent. P_cont is the rating the generator must hold indefinitely. P_surge is the peak it must supply for a few seconds while the largest motor starts with everything else already running [1].

Only the largest motor’s surge is counted, because the worst normal case is the biggest motor starting last. A motor draws several times its running power while it starts, so this one load usually sets the generator size [1].

Limits and common mistakes

• Enter the largest motor’s running watts in the total as well. The page adds only the surge above its running power.
• Starting watts are often two to three times the running watts, and more for a compressor that starts against pressure. Use the nameplate or the maker’s figure when you have it.
• The inputs are real power in watts. A generator rated in kVA delivers kVA × power factor in watts, often 0.8.
• If two large motors can start together, add both surges by hand. The page assumes one starts at a time.
• Altitude and heat lower a generator’s output. Check the maker’s derating.

Worked example

A house needs a backup generator for a well pump, a refrigerator, the furnace blower and some lights and electronics. They add up to 4200 W running. The well pump is the largest motor: 1000 W running and 2100 W starting.

1. Enter Total running load (all loads) 4200 W.
2. Enter Largest motor’s running watts (included above) 1000 W and Largest motor’s starting surge watts 2100 W.
3. Enter Safety margin (%) 10 and press Solve. The Solution shows: Minimum continuous rating: 4,620 W
   Minimum surge rating: 5,830 W

Check: 4200 × 1.1 = 4620 W. Starting the pump adds 2100 − 1000 = 1100 W, so the peak is 5300 W, and 5830 W with the margin. A generator rated about 5000 W running and 6000 W starting covers both.